[PATCH] [PPC64] No prefetch for NULL pointers

[Olof Johansson]

Hi,


For prefetches of NULL (as when walking a short linked list), PPC64 will
in some cases take a performance hit. The hardware needs to do the TLB
walk, and said walk will always miss, which means (up to) two L2 misses
as penalty. This seems to hurt overall performance, so for NULL pointers
skip the prefetch alltogether.

Signed-off-by: Olof Johansson <olof at austin.ibm.com>

Index: 2.6/include/asm-ppc64/processor.h
===================================================================
--- 2.6.orig/include/asm-ppc64/processor.h	2005-03-28 11:08:00.000000000 -0600
+++ 2.6/include/asm-ppc64/processor.h	2005-03-28 11:14:34.000000000 -0600
@@ -635,11 +635,17 @@ static inline unsigned long __pack_fe01(
 
 static inline void prefetch(const void *x)
 {
+	if (unlikely(!x))
+		return;
+
 	__asm__ __volatile__ ("dcbt 0,%0" : : "r" (x));
 }
 
 static inline void prefetchw(const void *x)
 {
+	if (unlikely(!x))
+		return;
+
 	__asm__ __volatile__ ("dcbtst 0,%0" : : "r" (x));
 }